The 200 so-called underground cities of Cappadocia -- dense elaborate networks carved into the soft rock of the Turkish region, capable of sheltering thousands of people -- may have been started as long ago as the 8th century BC.

Whoever built them failed to leave their initials on the walls but the Phrygians (an ancient Indo-European tribe) may have been responsible, with later expansion by the Persians.

It's known early Christians hid in the man-made caves while they were still being persecuted for their religion.

Derinkuyu, extending to a depth of 60 meters, is the largest cave -- it once housed 20,000 people, as well as their livestock.

Almost half of Derinkuyu is open to tourists. It's connected to another subterranean settlement you can explore, Kaymaklli, via an eight-kilometer tunnel.

The Shanghai Tunnels are a series of passages that connect the basements of many downtown Portland bars and hotels to the waterfront on the Willamette River.

Used prior to the 1800s to move goods from the ships that docked here, the tunnels got their name from the belief, probably false, that they were associated with "shanghaiing" -- the practice of kidnapping men to serve as sailors.

In the late 1950s, the U.S. government asked the owners of the 700-room Greenbrier Hotel, in West Virginia, for permission to build an emergency relocation center -- a bunker -- beneath the property, to house Congress in the event of a nuclear war.

Perhaps with some foresight (it's now one of West Virginia's most popular attractions), they agreed.

The bunker remained fully stocked for 30 years before being decommissioned in the early 1990s.

Daily tours of the facility inspect a 25-ton blast door, some of the 18 dormitories, a hospital and decontamination chambers.

The Cu Chi tunnels were used by the Viet Cong during large parts of the Vietnam War as living quarters, hospitals, supply routes and storage areas -- even a tank was found in one of the tunnels.

This 120-kilometer-long complex, part of a much larger network throughout the country, now operates as a war memorial and visitors -- or at least those who can squeeze through the tiny trapdoors -- can explore several of the tunnels.

The tunnels built beneath downtown Moose Jaw in the early 1900s were intended to protect Chinese railway workers from persecution during the so-called Yellow Peril, the racist panic whipped up about thousands of Asian immigrants who'd arrived to find work.

Entire immigrant families lived in the tunnels while working above ground, but during Prohibition the tunnels were used for smuggling.

Al Capone was among the smugglers believed to have used the tunnels -- daily tours are carried out by an Al Capone look-a-like.